Important functions of the radio receiver are such as near-by channel selectivity for eliminating near-by channels to demodulate only a desired wave, a function of preventing the radio receiver from operating as if a desired wave is received as a result of the generation of the same frequency component as that of desired wave by mixing of a plurality of disturbing waves, and a function of preventing interference by receiving channels of the same frequency with each other.
These functions are influenced by characteristics of various filters of a receiving high-frequency circuit and an intermediate-frequency amplifying circuit and the nonlinearity of respective circuits, and the degree of interference or disturbance differs with the correlation of receiving frequencies and the received signal level.
To make these functions satisfactory for practical use, efforts have been concentrated on the improvement of characteristics at the device level, such as enhancement of the linearity of the receiving high-frequency circuit and the intermediate-frequency amplifying circuit or maximization of the steepness of the filter characteristic at each stage.
Radio receivers, however, even if made sure to have such required functions through conventional measurement, are not always capable of good reception under the recent radio wave environment.
According to a prior art method for measuring the near-by channel selectivity of the radio receiver, the signal level of a near-by channel is raised in a state in which a signal is received at a level higher than a desired wave receiving sensitivity level of the receiving channel (that is, a state in which a desired wave whose SN ratio is, for example, 6 dB higher than a desired wave signal of a 20 dB is applied to the receiver) and a difference between the signal level of the nearby channel and the desired wave receiving level of the receiving channel is obtained when the above-said SN ratio drops by a predetermined value, and the value thus obtained is regarded as an indication of the near-by channel selectivity of the receiver.
Similar measurements may sometimes be made wherein two nearby waves or two waves bearing a predetermined frequency relationship are applied, as disturbing waves, to the receiver at the same level.
At any rate, conventional measurements of receiver functions all utilize the desired wave signal of the sensitivity level at the basis of measurement, as typified by the above method; hence, the absolute value of the level of the disturbing wave is inevitably very low in the evaluation of the functions.
However, there are cases where radio receivers, even if found by the above-mentioned measurement to possess the prescribed functions, are incapable of normal reception of signals of extremely high levels, even if the level difference between the desired wave and the disturbing wave is less than a prescribed value.
For example, in the vicinity of a base station where a multifrequency transmitting antenna is provided, the desired wave and the disturbing wave are both received at an extremely high level, but even if the disturbing wave is slightly higher in level than the desired wave, that is, even if the level difference between them is only 10 dB or so, the level of the desired wave in the receiver is suppressed owing to the nonlinearity of the receiving high-frequency circuit--this markedly decreases the receiving sensitivity, hindering the normal reception.
Under such conditions, not only the near-by channel selectivity but also the interference eliminating function are impaired; for example, frequency components nearby the desired wave receiving channel, which are generated by harmonic waves of two or more disturbing waves present anywhere in the same receiving frequency band or a mixture thereof, cause interference and hence pose the same problem as mentioned above.
Such a problem is encountered more frequently with the recent increase in the numbers of radio channels and base stations or repeater stations; namely, radio service zones are formed everywhere in the center of a city, in particular, where many radio stations are established--this constitutes a serious obstacle to mobile communications.